As positive photoresist composition there are normally used compositions comprising an alkali-soluble resin and a naphthoquinonediazide compound as a photosensitive material. Example of such composition include novolak type phenol resins/naphthoquinonediazide-substituted compounds as disclosed in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470. Most typical examples of such composition include novolak resin composed of cresol-formaldehyde/trihydroxybenzophenone1,2-naphthoquinonediazidosulfoni c ester as disclosed in L. F. Thompson, Introduction to Microlithoqraphy, ASC Publishing Co., No. 219, pages 112 to 121.
The novolak resin thus used as a binder not only is soluble in an aqueous alkali solution without being swelled by the solution, but also forms an image-bearing film which can be used as an etching mask having high resistance particularly to plasma etching. Consequently, novolak resins are especially useful in this application of use. On the other hand, the naphthoquinonediazide compound, which is used as a photosensitive material, is unique in that it itself functions as a dissolution inhibitor to reduce the alkali solubility of the novolak resin while it decomposes upon irradiation with light to yield an alkali-soluble substance thereby serving to enhance, rather than reduce, the alkali solubility of the novolak resin. Thus, due to this large change in its property upon light irradiation, naphthoquinonediazide compounds are useful especially as a photosensitive material in positive photoresists.
From the standpoints described above, a large number of positive photoresists comprising a novolak resin and a photosensitive naphthoquinonediazide compound have so far been developed and put to practical use, and such resist materials have succeeded in giving sufficient results in processing for forming patterns having line widths as small as the order of 1.5 to 2 .mu.m.
However, the degree of integration in integrated circuits is becoming higher increasingly, and the production of substrates for semiconductor circuits, e.g., VLSI's (very large scale integrated circuit), has come to necessitate a processing for forming an ultrafine pattern having a line width of 1 .mu.m or smaller. The photoresists for use in this application are required to have a high sensitivity in view of a high resolving power, a high pattern reproducing accuracy to reproduce the image of an exposure mask and a high productivity. However, conventional photoresists described above could not meet these problems.
As techniques for improving the above problems, there are known photoresist compositions comprising a naphthoquinonediazide photosensitive material and an alkali-soluble resin comprising specific monomers, or an alkali-soluble resin comprising specific monomers and a low-molecular weight alkali dissolution accelerator. Examples thereof are shown below.
JP-B-2-51499 (the term "JP-B" as used herein refers to an "examined Japanese patent publication") discloses resins obtained by condensing from 30 to 90 molar ratio of m-cresol, from 5 to 40 molar ratio of p-cresol, and xylenol (exclusive of 2,5-xylenol) using an aldehyde as an alkali-soluble novolak resin.
However, the techniques disclosed in JP-B-2-51499 are not sufficient in the sensitivity, the resolving power and the defocus latitude.
JP-B-3-36420 discloses resins obtained from condensation monomers containing m-cresol and xylenol but not containing p-cresol and an aldehyde as an alkali-soluble novolak resin.
However, the techniques disclosed in JP-B-3-36420 are not sufficient in the defocus latitude.
JP-B-6-54386 discloses resins obtained by condensing from 30 to 90 mol % of m-cresol and phenols containing from 70 to 10 mol % of the compound represented by the following formula (I) (there is no disclosure about p-cresol): EQU (OH).sub.n --Ph--X.sub.m (I)
wherein X represents --CH.sub.3, --C.sub.2 H.sub.5, --C(CH.sub.3).sub.3, --CO.sub.2 CH.sub.3 or --CO.sub.2 C.sub.2 H.sub.5 ; n is from 1 to 3; m is from 1 to 3; and Ph represents a phenyl group with aldehydes as an alkali-soluble novolak resin.
However, the techniques disclosed in JP-B-6-54386 are not sufficient in the defocus latitude.
JP-B-3-54565 discloses resins obtained by condensing from 20 to 80 mol % of m-cresol, from 0 to 50 mol % of p-cresol, and phenols containing from 20 to 80 mol % of at least one compound represented by formula (I) other than m-cresol and p-cresol with aldehydes. However, the techniques disclosed in JP-B-3-54565 are not sufficient in the defocus latitude. Further, JP-B-3-54565 discloses inclusion of 2H-pyrido[3,2-b]-1,4-oxazin-3(4H)-ones to increase sensitivity, but there arises another problem of generation of scum.
JP-A-6-202321 discloses resist compositions comprising a novolak resin obtained by condensing a phenol mixture containing m-cresol, p-cresol, and 2,5-xylenol with an aldehyde, and an alkali-soluble compound having a molecular weight of less than 900. However, the techniques disclosed in JP-A-6-202321 are not sufficient in the sensitivity, the resolving power and the defocus latitude.
JP-A-4-122938 discloses photoresist compositions comprising an alkali-soluble phenol-novolak resin having the ratio of the weight average molecular weight to the number average molecular weight of from 1.5 to 4.0, and from 2 to 30 wt %, based on the novolak resin, of a low- molecular weight compound having from 2 to 8 phenolic hydroxyl groups per molecule. However, there are no suggestions about the specific monomers of the alkali-soluble phenol-novolak resins and the constitutional ratios thereof.
These techniques are also not sufficient and the defocus latitude is narrow, therefore, resist compositions of superior resist performances are still desired.